A hybrid circuit is always provided between a transmitter and a receiver, to respectively transform a transmitting signal and a receiving signal to desired voltage levels. FIG. 1 is a block diagram illustrating relations between a transmitter 101, a receiver 103 and a hybrid circuit 105 for the related art. As shown in FIG. 1, in an upstream operation, the hybrid circuit 105 receives a transmitting signal TS from the transmitter 101 via at least one transmitting lime Lt, and transforms the transmitting signal TS to a transformed transmitting signal TTS to at least one transceiving line Ltr. On the contrary, in a downstream operation, the hybrid circuit 105 receives a receiving signal RS via the transceiving line Ltr, and transforms the receiving signal RS to a transformed receiving signal TRS to the receiver 103 via at least one receiving line Lr.
FIG. 2 and FIG. 3 are circuit diagrams illustrating detail structures of a hybrid circuit for related art. FIG. 2 illustrates a parallel hybrid circuit for related art and FIG. 3 illustrates a serial hybrid circuit for related art. As shown in FIG. 2, the hybrid circuit 200 comprises a transformer 201 and two resistors R1, R2. The transformer 201 comprises a first side comprising a coil C11 with a first coil number cn1 and a coil C12 with the first coil number cn1, and a second side comprising a coil C21 with a second coil number cn2 and a coil C22 with the second coil number cn2.
For an upstream operation, a first terminal T1 of the coil C11 and a second terminal T2 of the coil C12 receive a transmitting signal TS. The transmitting signal TS is transformed from the first side to the second side via a first turn ratio equaling to cn1:cn2, to generate the transformed transmitting signal TTS.
For a downstream operation, a first terminal T1 of the coil C21 and a second terminal T1 of the coil C22 receive a receiving signal RS. The receiving signal RS is coupled from the second side to the first side via a second turn ratio equaling to cn2:cn1.
However, in such structure, the first turn ratio is cn1:cn2 and the second turn ratio is cn2:cn1, thus the second turn ratio is fixed a reciprocal of the first turn ratio. It is not convenient since the signal may be desired to be transformed to other voltage levels.
As above-mentioned, FIG. 3 is a circuit diagram illustrating a serial hybrid circuit for related art. In such related art, a first side of the transformer 301 comprises a coil C1 with a first coil number cn1 and a coil C3 with a third coil number cn3. Also, a second side of the transformer 301 comprises a coil C2 with a second coil number cn2.
For an upstream operation, the hybrid circuit 300 couples the transmitting signal TS from the first side to the second side to generate the transformed transmitting signal TTS via a first turn ratio. Additionally, for the downstream operation, the hybrid circuit 300 couples to the receiving signal RS from the second side to the first side to generate the transformed receiving signal TRS via a second turn ratio. In this structure, the first ratio equals to cn1:cn2, and the second ratio equals to cn2:cn3, therefore, the second turn ratio is not limited to a reciprocal of the first turn ratio. However, the structure of the hybrid circuit 300 comprises resistors R1 and R2 coupled with the transformer in series, thus a coil number for the upstream path must be high. Leakage inductance may exist for such structure, which causes recession for high frequency signals. Also, such structure may have a high echo cn1:cn3, which occurs between an upstream path and a down stream path.